1. Field of Invention
The present invention relates to a driver circuit of a display. More particularly, the present invention relates to a digital-to-analog converter (DAC) in the driver circuit.
2. Description of Related Art
Those skilled in the art know that various DAC architectures are available, such as a resistor string (R-string) DAC. FIG. 1 illustrates a conventional R-string DAC. An R-string DAC 100 includes a selector 122 and several resistors 112. By the selector 122, the connecting nodes between the resistors 112 constitute tap points selectively switched to an output node in response to a digital input. The voltage of one tap point, which is selected to couple to the output node, is an inherently monotonic analog representation of the digital input.
DACs are generally used in a driver circuit of an OLED (Organic Light Emission Display) to accomplish the modulations of brightness, contrast or gray scale for OLED pixels. Particularly, for a current-type AMOLED (Active-Matrix Organic Light Emission Display), the DAC finally should outputs a current rather than a voltage for driving the OLED pixel. For example, the DAC may be of a pure current-type architecture, which modulates its output by multiplying currents with transistors of different length-to-width ratios.
However, when the output of the DAC needs more variations, such as the digital input of more bits is applied or more gray scales are required in design, the transistors necessarily will occupy larger valuable area in the driver circuit. Alternatively, the DAC of a pure voltage-type architecture whose output voltage is subsequently converted into a current is very complex and hard to decide the tap point, increasing the difficulty of circuitry design.